Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("请输入你要计算的数字：");
            long n = Convert.ToInt64(Console.ReadLine());
            Console.Write("{0}的阶乘是:", n);
            Console.WriteLine(GetResult(n));
        }
        static string GetResult(long n)
        {
            List<long> listResult = new List<long>();
            listResult.Add(1);

int posCount=1;//记录总位数
            for (long i = 1; i <= n; i++)
            {
                long carry = 0;
                for (int j = 0; j < posCount; j++)
                {
                    
                    //先相乘
                    listResult[j] *= i;  
                    //加上进位
                    listResult[j] += carry;
                    long temp = listResult[j];
                    //存储个位数
                    listResult[j] %= 10;
                    carry = temp / 10;
                    if (carry > 0 && j==listResult.Count-1)
                    {
                        posCount++;
                        listResult.Add(0);
                    }
                }
            }

StringBuilder sb = new StringBuilder();
            for (int i = listResult.Count-1; i >=0 ; i--)
            {
                sb.Append(listResult[i].ToString());
            }
            return sb.ToString();
        }
    }
}